I began working with PLC's at
a major Japanese manufacturer of office equipment in based in Shropshire.
There were many PLC's installed in various production lines, assembly
equipment and robots. I installed Omron PLC's into a fleet of Automatically
Guided Vehicles that I designed. The AGV's carried photocopiers around the
plant, automatically transferring between one production line and the next.
The PLC installed on board took care of managing the route to take and what
to do when it got there. The AGV PLC communicated to the production line
PLC's in order to instigate and manage transfer from the vehicle to the
conveyor, the PLC also commanded the motion controllers which took
care of the drive and differential steering. Another PLC was statically
based and kept track of each AGV in the fleet, effectively managing the
whole system. This was quite a first PLC project, eventually saving the
company over £300,000 against a similar system bought from their usual
supplier. After 10 years and studying ONC and HNC I moved on to a new
position. PLC based Special purpose machinery for the rubber and plastic
industries. Most equipment went into tyre (tire) plants all over the
world. I designed PLC control systems , wrote the PLC and motion
control software, installed it and commissioned in house and on site all
over the world. This was an interesting position with the great opportunity
to travel the world while still being involved with PLC control systems.
Some small machines such as Tube splicers were installed with various brands
of brick PLC as specified by the customer, the larger machines such as Tire
builders generally had modular PLC's such as Allen Bradley SLC505. The fully
automated bias cutter machines with PLC I/O counts of over 400 had modular
PLC's with distributed I/O and SCADA systems. I installed and serviced PLC
based tire machinery in the UK, USA, Canada, India, China, Indonesia and got
to meet some great people. When I started my own PLC control company I
continued to work all over the world but in many different industries, I
have installed PLC control systems in Breweries, Power stations, Potato
processing plants, steel plants, rubber plants, chemical processing plants
and many more, all over the world. As with any technology PLC's progress and
PLC's installed 10 or 15 years ago may not be operating your machinery
to the optimum, with increased flexibility in good PLC systems such as
integrated motion control, increases in quality and efficiency can be
achieved. Replacing an outdated control system, PLC with an upto
date PLC control system can yield significant benefits.

About PLC's

The main difference from other computers is that PLCs are armored for severe conditions (dust,
moisture, heat, cold, etc) and have the facility for extensive input/output
(I/O) arrangements. These connect the PLC to sensors and actuators. PLCs
read limit switches, analog process variables (such as temperature and
pressure), and the positions of complex positioning systems. Some even use
machine vision. On the actuator side, PLCs operate electric motors,
pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog
outputs. The input/output arrangements may be built into a simple PLC, or
the PLC may have external I/O modules attached to a computer network that
plugs into the PLC. System scale
A small PLC will have a
fixed number of connections built in for inputs and outputs. Typically,
expansions are available if the base model does not have enough I/O.Modular PLCs have a
chassis (also called a rack) into which are placed modules with different
functions. The processor and selection of I/O modules is customised for the
particular application. Several racks can be administered by a single
processor, and may have thousands of inputs and outputs. A special high
speed serial I/O link is used so that racks can be distributed away from the
processor, reducing the wiring costs for large plants.PLCs may need to
interact with people for the purpose of configuration, alarm reporting or
everyday controlA Human-Machine
Interface (HMI) is employed for this purpose. HMIs are also referred to as
MMIs (Man Machine Interface) and GUI (Graphical User Interface).A simple system may use
buttons and lights to interact with the user. Text displays are available as
well as graphical touch screens. More complex systems use a programming and
monitoring software installed on a computer, with the PLC connected via a
communication interface.CommunicationsPLCs have built in
communications ports usually 9-Pin RS232, and optionally for RS485 and
Ethernet. Modbus or DF1 is usually included as one of the communications
protocols. Others' options include various fieldbuses such as DeviceNet or
Profibus. Other communications protocols that may be used are listed in the
List of automation protocols.Most modern PLCs can
communicate over a network to some other system, such as a computer running
a SCADA (Supervisory Control And Data Acquisition) system or web browser.PLCs used in larger I/O
systems may have peer-to-peer (P2P) communication between processors. This
allows separate parts of a complex process to have individual control while
allowing the subsystems to co-ordinate over the communication link. These
communication links are also often used for HMI (Human-Machine Interface)
devices such as keypads or PC-type workstations. Some of today's PLCs can
communicate over a wide range of media including RS-485, Coaxial, and even
Ethernet for I/O control at network speeds up to 100 Mbit/s.PLC compared with other control systemsPLCs are well-adapted
to a range of automation tasks. These are typically industrial processes in
manufacturing where the cost of developing and maintaining the automation
system is high relative to the total cost of the automation, and where
changes to the system would be expected during its operational life. PLCs
contain input and output devices compatible with industrial pilot devices
and controls; little electrical design is required, and the design problem
centers on expressing the desired sequence of operations in ladder logic (or
function chart) notation. PLC applications are typically highly customized
systems so the cost of a packaged PLC is low compared to the cost of a
specific custom-built controller design. On the other hand, in the case of
mass-produced goods, customized control systems are economic due to the
lower cost of the components, which can be optimally chosen instead of a
"generic" solution, and where the non-recurring engineering charges are
spread over thousands or millions of units.For high volume or very
simple fixed automation tasks, different techniques are used. For example, a
consumer dishwasher would be controlled by an electromechanical cam timer
costing only a few dollars in production quantities.A microcontroller-based
design would be appropriate where hundreds or thousands of units will be
produced and so the development cost (design of power supplies and
input/output hardware) can be spread over many sales, and where the end-user
would not need to alter the control. Automotive applications are an example;
millions of units are built each year, and very few end-users alter the
programming of these controllers. However, some specialty vehicles such as
transit busses economically use PLCs instead of custom-designed controls,
because the volumes are low and the development cost would be uneconomic.Very complex process
control, such as used in the chemical industry, may require algorithms and
performance beyond the capability of even high-performance PLCs. Very
high-speed or precision controls may also require customized solutions; for
example, aircraft flight controls.Programmable
controllers are widely used in motion control, positioning control and
torque control. Some manufacturers produce motion control units to be
integrated with PLC so that G-code (involving a CNC machine) can be used to
instruct machine movements.PLCs may include logic
for single-variable feedback analog control loop, a "proportional, integral,
derivative" or "PID controller." A PID loop could be used to control the
temperature of a manufacturing process, for example. Historically PLCs were
usually configured with only a few analog control loops; where processes
required hundreds or thousands of loops, a distributed control system (DCS)
would instead be used. However, as PLCs have become more powerful, the
boundary between DCS and PLC applications has become less clear-cut.PLCs have similar
functionality as Remote Terminal Units. An RTU, however, usually does not
support control algorithms or control loops. As hardware rapidly becomes
more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to
overlap in responsibilities, and many vendors sell RTUs with PLC-like
features and vice versa. The industry has standardized on the IEC 61131-3
functional block language for creating programs to run on RTUs and PLCs,
although nearly all vendors also offer proprietary alternatives and
associated development environments.Digital and analog signalsDigital or discrete
signals behave as binary switches, yielding simply an On or Off signal (1 or
0, True or False, respectively). Push buttons, limit switches, and
photoelectric sensors are examples of devices providing a discrete signal.
Discrete signals are sent using either voltage or current, where a specific
range is designated as On and another as Off. For example, a
PLC might use 24 V DC I/O, with values above 22 V DC representing On,
values below 2VDC representing Off, and intermediate values
undefined. Initially, PLCs had only discrete I/O.Analog signals are like
volume controls, with a range of values between zero and full-scale. These
are typically interpreted as integer values (counts) by the PLC, with
various ranges of accuracy depending on the device and the number of bits
available to store the data. As PLCs typically use 16-bit signed binary
processors, the integer values are limited between -32,768 and +32,767.
Pressure, temperature, flow, and weight are often represented by analog
signals. Analog signals can use voltage or current with a magnitude
proportional to the value of the process signal. For example, an analog 4-20
mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.Current inputs are less
sensitive to electrical noise (i.e. from welders or electric motor starts)
than voltage inputs.As an example, say a
facility needs to store water in a tank. The water is drawn from the tank by
another system, as needed, and our example system must manage the water
level in the tank.Using only digital
signals, the PLC has two digital inputs from float switches (Low Level and
High Level). When the water level is above the switch it closes a contact
and passes a signal to an input. The PLC uses a digital output to open and
close the inlet valve into the tank.When the water level
drops enough so that the Low Level float switch is off (down), the PLC will
open the valve to let more water in. Once the water level rises enough so
that the High Level switch is on (up), the PLC will shut the inlet to stop
the water from overflowing. This rung is an example of seal in logic. The
output is sealed in until some condition breaks the circuit.An analog system might
use a water pressure sensor or a load cell, and an adjustable (throttling)
dripping out of the tank, the valve adjusts to slowly drip water back into
the tank.In this system, to
avoid 'flutter' adjustments that can wear out the valve, many PLCs
incorporate "hysteresis" which essentially creates a "deadband" of activity.
A technician adjusts this deadband so the valve moves only for a significant
change in rate. This will in turn minimize the motion of the valve, and
reduce its wear.A real system might
combine both approaches, using float switches and simple valves to prevent
spills, and a rate sensor and rate valve to optimize refill rates and
prevent water hammer. Backup and maintenance methods can make a real system
very complicated.PLC programs are
typically written in a special application on a personal computer, then
downloaded by a direct-connection cable or over a network to the PLC. The
program is stored in the PLC either in battery-backed-up RAM or some other
non-volatile flash memory. Often, a single PLC can be programmed to replace
thousands of relays.Under the IEC 61131-3
standard, PLCs can be programmed using standards-based programming
languages. A graphical programming notation called Sequential Function
Charts is available on certain programmable controllers.Recently, the
International standard IEC 61131-3 has become popular. IEC 61131-3 currently
defines five programming languages for programmable control systems: FBD
(Function block diagram), LD (Ladder diagram), ST (Structured text, similar
to the Pascal programming language), IL (Instruction list, similar to
assembly language) and SFC (Sequential function chart). These techniques
emphasize logical organization of operations.hile the fundamental
concepts of PLC programming are common to all manufacturers, differences in
I/O addressing, memory organization and instruction sets mean that PLC
programs are never perfectly interchangeable between different makers. Even
within the same product line of a single manufacturer, different models may
not be directly compatible.The PLC was invented in
response to the needs of the American automotive manufacturing industry.
Programmable controllers were initially adopted by the automotive industry
where software revision replaced the re-wiring of hard-wired control panels
when production models changed.Before the PLC,
control, sequencing, and safety interlock logic for manufacturing
automobiles was accomplished using hundreds or thousands of relays, cam
timers, and drum sequencers and dedicated closed-loop controllers. The
process for updating such facilities for the yearly model change-over was
very time consuming and expensive, as the relay systems needed to be rewired
by skilled electricians.In 1968 GM Hydramatic
(the automatic transmission division of General Motors) issued a request for
proposal for an electronic replacement for hard-wired relay systems.The winning proposal
came from Bedford Associates of Bedford, Massachusetts. The first PLC,
designated the 084 because it was Bedford Associates' eighty-fourth project,
was the result. Bedford Associates started a new company dedicated to
developing, manufacturing, selling, and servicing this new product: Modicon,
which stood for MOdular DIgital CONtroller. One of the people who worked on
that project was Dick Morley, who is considered to be the "father" of the
PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later
acquired by German Company AEG and then by French Schneider Electric, the
current owner.One of the very first
084 models built is now on display at Modicon's headquarters in North
Andover, Massachusetts. It was presented to Modicon by GM, when the unit was
retired after nearly twenty years of uninterrupted service. Modicon used the
84 moniker at the end of its product range until the 984 made its
appearance.The automotive industry
is still one of the largest users of PLCs.Early PLCs were
designed to replace relay logic systems. These PLCs were programmed in
"ladder logic", which strongly resembles a schematic diagram of relay logic.
Modern PLCs can be programmed in a variety of ways, from ladder logic to
more traditional programming languages such as BASIC and C. Another method
is State Logic, a Very High Level Programming Language designed to program
PLCs based on State Transition Diagrams.Many of the earliest
PLCs expressed all decision making logic in simple ladder logic which
appeared similar to electrical schematic diagrams. This program notation was
chosen to reduce training demands for the existing technicians. Other early
PLCs used a form of instruction list programming, based on a stack-based
logic solver.Early PLCs, up to the
mid-1980s, were programmed using proprietary programming panels or
special-purpose programming terminals, which often had dedicated function
keys representing the various logical elements of PLC programs. Programs
were stored on cassette tape cartridges. Facilities for printing and
documentation were very minimal due to lack of memory capacity. The very
oldest PLCs used non-volatile magnetic core memory.The functionality of
the PLC has evolved over the years to include sequential relay control,
motion control, process control, distributed control systems and networking.
The data handling, storage, processing power and communication capabilities
of some modern PLCs are approximately equivalent to desktop computers.
PLC-like programming combined with remote I/O hardware, allow a
general-purpose desktop computer to overlap some PLCs in certain
applications.
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